H6.1 - OSPF Routing Protocol Brief.pdf

H6.1 - OSPF Routing Protocol Brief.pdf - Cisco OSPF Routing...

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OSPF Routing Protocol Contents Introduction Network Architecture Campus Design Architecture Building Block Design Server Farm Design Core Block Design WAN Design Architecture Protocol Design Campus Design Considerations Layer 2 versus Layer 3 Core Design WAN Design Considerations Design Recommendation Summary Planning and Implementation IP Addressing Summarization Scalability Migration Convergence Tuning Design Case Study Operating the Solution Operation Verification Troubleshooting Related Information Introduction Open Shortest Path First (OSPF) is an interior gateway protocol (IGP) link state protocol. Contrary to the distance vector protocol in which the actual Internet Protocol (IP) network is advertised periodically, in a link state protocol there is no IP route exchange. Every participant router creates a Link State Advertisement (LSA) describing its local interface (IP address, network mask, reachable neighbor, link type, and so on) and places it in its database. LSAs are distributed through reliable flooding during database synchronization, and the collection of all LSAs constitute a link-state database. Cisco - OSPF Routing Protocol http://www.cisco.com/partner/sdm/ci/routing/ospf/CI_OSPF.html (1 of 48) [10/11/2001 5:34:43 PM]
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All routers within an area have the exact same link state database and run in parallel with the shortest path or Dijkstra algorithm . Each router constructs a tree of shortest path with itself as a root. The shortest path tree gives the route to all destinations within the autonomous system. Compared to distance vector protocols that have a flat architecture, OSPF uses a hierarchical architecture. By having a hierarchical design, routing control packets in the domain are decreased and limited to a given area. In addition, summarization between different hierarchical levels significantly increases the stability of the network and decreases the size of the routing table. OSPF allows a network to be segmented into multiple areas. An area is a collection of routers and networks. All areas are attached, physically or logically, to a common area called the backbone area (area 0). Routing between areas is achieved through area 0, and summarization occurs at Area Border Routers (ABRs) that are attached to the backbone area 0 and another non-backbone area. The three components in OSPF include: Neighbor discovery Database synchronization Shortest Path First (SPF) calculation Router Classifications There are four types of routing nodes in OSPF. Each routing node provides a specific function. Internal router—A router that has all its interfaces in a given area ABR (Area Border Router)—A router that has active interfaces in at least two areas, one being the backbone area 0 ASBR (Autonomous System Border Router)—A router that injects external routes into the OSPF domain by redistributing any routing protocol or external route to the OSPF domain is known as an ASBR
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